Long Term Evolution (LTE) networks use Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink and DFT-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms. For a normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each OFDM symbol is approximately 71.4 μs.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of e.g., the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
From LTE Rel-11 onwards, resource assignments can be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). For Rel-8 to Rel-10 only the Physical Downlink Control Channel (PDCCH) is available.
The reference symbols shown in FIG. 3 are the cell specific reference symbols (CRS) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
In a cellular communication system there is a need to measure the channel conditions in order to know what transmission parameters to use. These parameters include, e.g., modulation type, coding rate, transmission rank, and frequency allocation. This applies to uplink (UL) as well as downlink (DL) transmissions.
The scheduler that makes the decisions on the transmission parameters is typically located in the base station (eNB). Hence, it can measure channel properties of the UL directly using known reference signals that the terminals (user equipment or “UEs”) transmit. These measurements then form a basis for the UL scheduling decisions that the eNB makes, which are then sent to the UEs via a downlink control channel.
However, for the DL the eNB is unable to measure any channel parameters. Rather, it must rely on information that the UEs can gather and subsequently send back to the eNB. This so-called Channel-State Information (CSI) is obtained in the UEs by measuring on known reference symbols, Channel-State Information Reference Symbols (CSI-RS), transmitted in the DL. See the 3GPP specification 36.211, which pertains to LTE specifically.
The CSI-RS are UE specifically configured by radio resource signaling (RRC), with a certain configured periodicity, T={5, 10, 20, 40, 80} ms (i.e., every Tth subframe). There is a possibility to configure both non-zero power (NZP) CSI-RS and zero power (ZP) CSI-RS, where the ZP CSI-RS is simply an unused resource that can be matched to a NZP CSI-RS in an adjacent eNB. This will improve the SINR for the CSI-RS measurements for a UE served by the adjacent cell. The ZP CSI-RS can also be used as CSI-IM as introduced in Rel.11 and explained below.
In LTE, the format of the CSI reports are specified in detail and may contain CQI (Channel-Quality Information), a Rank Indicator (RI), and a Precoding Matrix Indicator. See 3GPP Specification 36.213. The CSI reports can be wideband or applicable to subbands. They can be configured by a radio resource control (RRC) message to be sent periodically or in an aperiodic manner, triggered by a control message from the eNB to a UE. The quality and reliability of the CSI are crucial for the eNB in order to make the best possible scheduling decisions for upcoming DL transmissions.
The LTE standard does not specify in detail how the UE should obtain and average these measurements from multiple time instants. For example, the UE may measure over a time frame unknown to the eNB and combine several measurements in a UE-proprietary way to create the CSI-values that are reported, either periodically or triggered.
In the context of LTE, the available CSI-RS are referred to as “CSI-RS resources”. In addition, there are also “CSI-IM resources”, where IM stands for “Interference Measurement”. The latter are defined from the same set of possible physical locations in the time/frequency grid as the CSI-RS, but with zero power, hence “ZP CSI-RS.” In other words, they are “silent” CSI-RS and when the eNB is transmitting the shared data channel, it avoids mapping data to those resource elements used for CSI-IM. These are intended to give a UE the possibility to measure the power of any interference from another transmitter than its serving node.
Each UE can be configured with one, three, or four different CSI processes. Each CSI process is associated with one CSI-RS and one CSI-IM where these CSI-RS resources have been configured to the UE by RRC signaling and are thus periodically transmitted/occurring with a periodicity of T and with a given subframe offset relative to the frame start.
If only one CSI process is used, then it is common to let the CSI-IM reflect the interference from transmission in all other eNBs except the eNB serving the UE, i.e., the serving cell uses a ZP CSI-RS that overlaps with the CSI-IM configured to the UE, but in other adjacent eNBs, there is no ZP CSI-RS on these resource elements. In this way, the UE will measure the interference from adjacent cells when measuring the received power in the resource elements configured as CSI-IM.
If additional CSI processes are configured to the UE, then there is a possibility for the network to also configure a ZP CSI-RS in the adjacent eNB that overlaps with a CSI-IM for this CSI process for the UE in the serving eNB. In this way the UE will send accurate CSI feedback also for the case when this adjacent cell is not transmitting. Hence, CSI feedback to support coordinated scheduling between eNBs is enabled with the use of multiple CSI processes where a first CSI process can be configured to the UE to feed back CSI for the full interference case and a second CSI process feeds back CSI for the case when a (preferably a strong interfering) adjacent cell is muted. Hence, the eNB receives CSI feedback for two possible transmission hypotheses and will then use this in the coordinated scheduler. As mentioned above, up to four CSI processes can be configured to the UE, thereby enabling feedback of four different transmission hypotheses.
Physical Downlink Control Channel (PDCCH) and Enhanced PDCCH (EPDCCH)
The PDCCH/EPDCCH is used to carry downlink control information (DCI) such as scheduling decisions and power-control commands. More specifically, the DCI includes:                Downlink scheduling assignments, including PDSCH resource indication, transport format, hybrid-ARQ information, and control information related to spatial multiplexing (if applicable). A downlink scheduling assignment also includes a command for power control of the PUCCH used for transmission of hybrid-ARQ acknowledgements in response to downlink scheduling assignments.        Uplink scheduling grants, including PUSCH resource indication, transport format, and hybrid-ARQ-related information. An uplink scheduling grant also includes a command for power control of the PUSCH.        Power-control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants.        
One PDCCH/EPDCCH carries one DCI message containing one of the groups of information listed above. As multiple terminals can be scheduled simultaneously, and each terminal can be scheduled on both downlink and uplink simultaneously, there must be a possibility to transmit multiple scheduling messages within each subframe. Each scheduling message is transmitted on separate PDCCH/EPDCCH resources, and consequently there are typically multiple simultaneous PDCCH/EPDCCH transmissions within each subframe in each cell. Furthermore, to support different radio-channel conditions, link adaptation can be used, where the code rate of the PDCCH/EPDCCH is selected by adapting the resource usage for the PDCCH/EPDCCH, to match the radio-channel conditions.
Carrier Aggregation
The LTE Rel-10 standard supports bandwidths larger than 20 MHz. One important requirement on LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This should also include spectrum compatibility. That would imply that an LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e., that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CCs, where the CCs have, or at least the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 4.
The number of aggregated CCs as well as the bandwidth of the individual CCs may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in the downlink and uplink is the same, whereas an asymmetric configuration refers to the case that the number of CCs is different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal. A terminal may for example support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
Cross-Carrier Scheduling
Scheduling of a CC is done on the PDCCH or EPDCCH via downlink assignments. Control information on the PDCCH/EPDCCH is formatted as a Downlink Control Information (DCI) message. In Rel-8 a terminal only operates with one DL and one UL CC. The association between DL assignment, UL grants, and the corresponding DL and UL CCs is therefore clear. In Rel-10 two modes of CA needs to be distinguished. The first case is very similar to the operation of multiple Rel-8 terminals, a DL assignment or UL grant contained in a DCI message transmitted on a CC is either valid for the DL CC itself or for an associated (either via cell-specific or UE specific linking) UL CC. A second mode of operation augments a DCI message with the Carrier Indicator Field (CIF). A DCI containing a DL assignment with CIF is valid for that DL CC indicated with CIF and a DCI containing an UL grant with CIF is valid for the indicated UL CC. The DCI transmitted using EPDCCH which was introduced in Rel-11 can also carry CIF which means that cross carrier scheduling is supported also when using EPDCCH.
Licensed Assisted Access (LAA) to Unlicensed Spectrum Using LTE
Up to now, the spectrum used by LTE is dedicated to LTE. This has the advantage that LTE systems do not need to care about the coexistence issue, and the spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited, which cannot meet the ever increasing demand for larger throughput from applications/services. Therefore, discussions are ongoing in 3GPP to initiate a new study item on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, LTE needs to consider the coexistence issue with other systems such as IEEE 802.11 (Wi-Fi). Operating LTE in the same manner in unlicensed spectrum as in licensed spectrum can seriously degrade the performance of Wi-Fi as Wi-Fi will not transmit once it detects that a channel is occupied.
Furthermore, one way to utilize the unlicensed spectrum reliably is to transmit essential control signals and channels on a licensed carrier. That is, as shown in FIG. 5, a UE is connected to a PCell in the licensed band and one or more SCells in the unlicensed band. In this application we denote a secondary cell in unlicensed spectrum as a “license assisted secondary cell” (LA SCell).
Small Cells ON/OFF
One of the mechanisms for interference avoidance and coordination among small cells is the small cell on/off feature. According to this feature the small cell may be turned on and off where the “on” and “off” period may depend on the criteria or application. Another purpose of small cell on/off can be for energy saving.
Discovery Signals
In LTE Rel-12, for small cell on/off where the eNB can be off for long periods of time, in order to assist the UE with the measurements, a discovery signal might be needed. The discovery signal needs to support the properties required for enabling RRM measurements, RLM related procedures, and coarse time/frequency synchronization. In order to make UE measurements possible, the eNB has to wake up periodically (e.g., once every 80 ms, or 160 ms, etc.) and send the discovery signal so that it can be used by the UE for mobility related operations such as cell identification, RLM, and measurement.
Within one cell, there may be multiple transmission points, from which the downlink signal can transmitted. Examples of this are a distributed antenna system, where multiple radio remote heads that are physically dislocated within the cell, transmit signals that all belong to the same cell (i.e., same CellID). The term transmission point may also refer to a sector of a site where the different sectors of the same site then constitute different transmission points. The discovery signal should also be capable of identifying individual transmission points and enabling RRM measurements for them.
In 3GPP, it has been agreed that the discovery signals will contain the PSS, SSS, and the CRS. The CSI-RS may optionally be present when configured. The discovery signal can be transmitted at least with the periodicities of 40, 80, and 160 ms. The duration of the discovery signal will be 5 subframes or less.
The discovery signal parameters will be communicated to the UE by the network node in order to enable the UE to make measurements. These parameters may include, e.g., a duration of each occasion when the discovery signal is transmitted (referred to as the DRS occasion), a periodicity of the occurrence of the occasion of discovery signals, a timing (in terms of a start time or an offset from a reference point such as a particular frame and subframe number) of the discovery signal occasions. In addition to the timing, periodicity and duration of the DRS occasion, and the discovery measurement timing configuration (DMTC) may also be signaled to the UE which tells the UE which subframes should be used to make measurements on the discovery signal. The signaling of the DMTC is currently being discussed in 3GPP.
DRX (Discontinuous Reception)
DRX is a RRC configured mechanism which enables the UE to save power during a cycle, referred to as the DRX cycle. When the UE is in DRX, it can sleep by relaxing its receiver for the remaining portion of the DRX cycle. The time that the UE is not in DRX can be considered as the “Active Time.” In the following, some important aspects of the DRX mechanism are described.
When a UE is configured with a (long) DRX cycle, it monitors the downlink control channel in some configured subframes during a DRX cycle referred to as the “onDuration,” where if the UE fails to decode any DL transmission, it goes to DRX. In case the UE decodes DL transmission, it comes out of DRX by starting a timer, referred to as the “drx-InactivityTimer” and continues monitoring the downlink control channel until the timer is expired. This timer starts or restarts with decoding any new DL assignment. When the drx-InactivityTimer expires the UE returns to DRX and sleeps until the next onDuration of the DRX cycle. Additionally, the DL and UL HARQ retransmissions occur as usual irrespective of the DRX cycle.
Moreover, two types of DRX cycles are introduced in the standards namely as the long and short DRX cycles, where the long cycle should be a multiple integer of the short cycle. A UE configured with the long DRX cycle can be also configured with a short DRX cycle. Such a UE, when coming out of long DRX, uses the short DRX cycle for a configured number of cycles and can fall back again to the long DRX cycle. An example of this is shown in FIG. 6, which is an example illustration of DRX timing event for a UE configured with both long and short DRX cycles only considering the DL control and data shared channel without the impact of the HARQ retransmissions, in which the variables “N” and “n” refer to the radio frame index and subframe index, respectively.
Assume that a UE is configured with DRX for the purpose of power saving. When the UE comes out of DRX, it monitors the (E)PDCCH during the Active Time where timers such as the onDuration Timer or the drx-InactivityTimer are started or restarted. The onDuration Timer starts or restarts as soon as the UE comes out of DRX and the drx-InactivityTimer starts or restarts as soon as the UE decodes an (E)PDCCH.
There is a potential problem in relation to the DRX procedure especially for groups of UEs with aligned onDuration periods that may particularly arise in high load scenarios which can result in system throughput performance degradation. When the load is high, it is probable that for groups of UEs with aligned onDuration, due to overloading of the control and shared data channels to support the current traffic, the scheduling assignments to some UEs could be postponed and earliest transmitted at some time beyond the onDuration of DRX cycle of those UEs. This implies that those UEs would return to DRX due to the lack of decoding of any scheduling assignment during the onDuration and consequently would wait another DRX cycle to check for a possible scheduling assignment which results in system throughput loss. The degradation is apparently more severe if such events occur when the UEs are using a long DRX cycle.
Another possible problem can occur in relation to UE behavior during the DRX procedure when DRX is operated alongside other procedures which rely on signaling to a group of UEs. The signaling to a group of UEs could be, for example, a physical layer signal which uses a particular DCI format. The physical layer signal or a physical layer signal to a group of UEs is transmitted from an eNB and contains some common information being shared among the UEs within the group. In general, the physical layer group signaling can be transmitted from cell X for the cell Y, i.e., the physical layer group signaling conveys information which corresponds to the UEs served by cell Y. The cells X and Y can be different (e.g., the physical layer group signaling can be transmitted from a PCell but for an SCell), or the cells X and Y can be the same (e.g., the physical layer signal can be transmitted from an SCell and be applicable to UEs in the same SCell). In the following, cell Y is referred to as the serving cell.
When group signaling occurs during the onDuration, and is decoded by the intended UEs, it forces all those UEs to remain active during their corresponding Active Time. However, the data traffic situation may differ for these UEs within the group. For example some of the UEs which have been forced to wake up due to the group signaling may not have any ongoing traffic but they have to continue monitoring the presence of scheduling assignments until their corresponding timer expires. This behavior results in an unnecessary increase in UE power consumption. This impact is further increased for such UEs if the short DRX cycle is configured because the UE has to go through the short DRX cycles until it can commence the long DRX cycle which means additionally increased power consumption due to more monitoring occasions in short DRX cycles.